The switched mode power supply (SMPS) is a well-known type of power converter having a diverse range of applications by virtue of its small size and weight and its high efficiency. For example, SMPSs are widely used in personal computers and portable electronic devices such as mobile phones. An SMPS achieves these advantages by switching a switching element, such as a power MOSFET, at a high frequency (usually tens to hundreds of kHz), with the frequency or duty cycle of the switching defining the efficiency with which an input voltage is converted to a desired output voltage.
FIG. 1 depicts a standard isolated SMPS with a number of switching devices Q1-Q6. The voltage across the primary side of the transformer T1 is controlled using switching devices Q1-Q4. Rectification of the voltage across the secondary side of the transformer T1 is achieved using switching devices Q5 and Q6.
As depicted in FIG. 1, the switching devices on the primary side of the isolated SMPS Q1-Q4 are in the full-bridge configuration. Other common primary side topologies are half-bridge and push-pull.
FIG. 1 also depicts a standard topology for the secondary side of transformer T1 in isolated SMPSs using a centre-tapped transformer. This yields full wave rectification using only two switching devices on the secondary side of the transformer T1 in contrast with a single secondary winding and full bridge using four switching devices for full-wave rectification. It is to be noted that the switching devices Q1-Q6 have an internal body drain diode which is not shown in the switching device symbol. The switching devices are not limited to the use of N-MOSFETs, P-MOSFETs or IGBT, as other types can be used.
A limitation of isolated SMPSs is that, to prevent saturation, the magnetic flux of the transformer must be kept balanced. Traditionally, to balance the magnetic flux, symmetrical duty cycles in a full switch cycle are used.
More particularly, a timing diagram for symmetric duty cycle switching is shown in FIG. 2. If D1 and D2 are the duty cycles for the switching device pairs Q1/Q4 and Q2/Q3, respectively then the switch period is denoted with T. In order to have the transformer magnetic flux balanced, the on-times for Q1/Q4 and Q2/Q3 should be identical in each switch period. Hence, the duty cycles D1 and D2 should be identical. A full switch cycle using such a scheme is set out below:    1 Time period: 0 to D1T/2: Q1/Q4 is conducting and energy transferred to the secondary side from the input source.    2. Time period: D1T/2 to T/2: Both Q5 and Q6 are conducting and the current is freewheeling through both the secondary side windings in order to have the transformer flux balanced.    3. Time period T/2 to T/2+D2T/2: Q2/Q3 is conducting and energy is transferred to the secondary side.    4. Time T/2+D2T/2 to T: same as in 2.
One problem, however, with balancing the magnetic flux using a symmetrical duty cycle is that the control loop must be run at half of the switching frequency, which yields a poor load transient response. That is, the constraint of maintaining the duty cycles, D1 and D2, as identical in a switch cycle yields a halved bandwidth of the output voltage control compared to a situation without this restriction. This yields a poor load transient response, which requires a large capacitive decoupling bank at the output in order to keep the voltage deviations during transients at acceptable levels.
On the other hand if the duty cycles are permitted to be asymmetric, so varying within the switch cycle, then the magnetic flux in the transformer must be balanced in another way, i.e., not only the output voltage but also the magnetic flux density must be regulated. This requires an increase in the controller complexity. An industry standard PID controller can not handle such a MIMO system.